1. Field of the Invention
The present invention relates to the field of die casting and more particularly to method and apparatus for the control of die casting machines.
2. Description of the Prior Art
In the art of die casting, one of the major problems encountered has been termed "gas entrapment". This problem is caused by the injection of air intermixed with molten material into a die which is used to form a casting. The intermixing of the air into the molten material causes such defects as blisters or internal cavities which may render the casting to be of inferior quality.
In the paper entitled "Metallographic Analysis of Zinc Die Castings", by Sanders et al, presented at the 6th SDCE International Die Casting Congress on Nov. 16, 1970, the effect of gas entrapment was recognized as the cause of blisters at the surface of die castings. Gas entrapment was also postulated as the cause of flow lines which appear as boundaries between distinct regions on the surface of the casting.
A proposal for overcoming the gas entrapment problem was offered in a paper entitled "Optimum Utilization of Die Casting Machines With Metrology", by James I. Moore, presented at the 8th SDCE International Die Casting Exposition and Congress, Mar. 17, 1975. The solution constituted a preprogrammed sequence of steps wherein a predetermined volume of molten metal "shot" was ladled into an injection tube containing a speed controlled plunger for ramming the metal into the die. The initial speed at which the plunger advanced, called the "shot speed", was relatively slow and preselected so that a wavefront developed in the molten material and tended to evacuate the air from the injection tube, through the die. After a predetermined time period, the plunger was then driven at a fast shot speed in order to inject the molten metal into the die.
The major disadvantage of the above solution was that it required a rather lengthy set-up time for each change in materials or dies. The set-up included a significant amount of experimentation to select the correct slow shot speed for a proper wave profile and determine the precise time in each injection cycle at which the change is made to the high shot speed. Of course, a second disadvantage was that the volume of molten material ladled into the cold chamber was also predetermined so that the predetermined time period would remain constant.